Recently, the involvement of various transporters localized on the plasma membrane in the uptake system for nutrients and endogenous substances into cells and their transport mechanisms have been clarified (Tsuji et al., Pharm. Res. 13:963–977, 1996). These transporters recognize the structures of substances to be transported to selectively transport specific substances across biological membranes. Transporters that recognize structures of a relatively wide range may possibly recognize foreign substances, such as drugs, by mistake, and actively take in them into cells. It is believed that drugs permeate through the plasma membrane fundamentally by simple diffusion, depending on their physicochemical properties such as molecular size, fat-solubility, and hydrogen-binding capacity. Particularly, according to the pH partition hypothesis, in the case of ionic drugs, only molecules in the non-dissociated form can permeate through the plasma membrane. However, it has become evident that a number of drugs penetrate through the cell membrane by a specific mechanism other than simple diffusion, that is, an active transport mediated by transporters, particularly in organs that require efficient exchange of intracellular and extracellular substances, including small intestine, uriniferous tubule, placenta, epithelial cells of choroid plexus, hepatocytes, and blood-brain barrier (Tamai et al., Pharmacia 31:493–497, 1995; Saito et al., Igaku no Ayumi 179:393–397, 1996; Tamai, I., Yakubutsu Dotai (Pharmacokinetics) 11:642–650, 1996). For example, it is known that although oral β-lactam antibiotics of the non-esterified type are amphoteric or negatively charged in physiological pHs and sparingly soluble in fat, they are readily absorbed through the intestine. A transport study using the isolated membrane-vesicles system demonstrated that an H+-driven peptide transporter localized on the brush-border membrane is involved in the absorption process of these drugs (Okano et al., J. Biol. Chem. 261:14130–14134, 1986). Although the specificity of a peptide transport system in terms of the peptide size is so strict as to recognize di- or tri-peptides but not tetrapeptides or larger peptides, it has a rather broad substrate specificity so as to recognize peptides comprising non-natural amino acids. The peptide transporter mistakenly mediates transport of β-lactam antibiotics due to its broad substrate specificity. This property has been unexpectedly utilized in the clinical field (Tsuji, A., American Chemical Society (eds. Taylor, M. D., Amidon, G. L.), Washington, D.C., 101–134, 1995). Furthermore, it has been reported that a transporter is possibly also involved in permeation of substances with a high fat-solubility such as fatty acids through the plasma membrane (Schaffer et al., Cell 79:427–436, 1994).
Since various transporters are presumed to be distributed in organs and cells based on the physiological roles of the organs and cells, their distribution and functions may be specific to organs. Therefore, transporters are expected to be used to impart an organ specificity to pharmacokinetics. In other words, an organ-specific drug delivery system (DDS) can be constructed utilizing transporters. If drug absorption that relies solely on simple diffusion is improved by elevating its fat-solubility, the effect of the drug obtained in the initial transport in the liver can be enhanced and the drug can non-specifically translocate into any organ. In addition, it would also be possible to increase the drug absorption independently of its fat-solubility by designing the drug based on the substrate specificity of transporters (Hayashi et al., Drug Delivery System 11:205–213, 1996). For this purpose, it is necessary to identify various transporters at the molecular level and analyze their properties in detail. However, molecular level identification is greatly behind studies on membrane physiology because the transporters are difficult to handle biochemically and require complicated processes in their functional assays.
Recently, cDNAs of several transporters have been cloned by the expression cloning method using Xenopus oocytes, a foreign gene expression system, and the structural homology among them has been revealed (Fei et al., Nature 368:563–566, 1994). For example, Koepsell et al. cloned an organic cation transporter, OCT1, which is presumed to be localized on a basement membrane, using the expression cloning method in 1994 (Grundemann et al., Nature 372:549–552, 1994). Subsequently, OCT2 was identified by homology cloning based on the sequence of OCT1 (Okuda et al., Biochem. Biophys. Res. Commun. 224:500–507, 1996). OCT1 and OCT2 show homology as high as 67% to each other (Grundemann et al., J. Biol. Chem. 272:10408–10413, 1997). Both of them are intensely express in the kindey, but differ in the organ distribution; OCT1 is also expressed in the liver, colon, and small intestine, while OCT2 expression is specific to the kidney.
In addition, another transporter, the human OATP transporter (hereinafter, referred to as “OATP-A”; Gastroenterology 109(4):1274–1282, 1995), has been reported. This transporter is a protein capable of transporting various endogenous and foreign substances in a sodium ion-independent manner. Known substances transported by OATP-A include bromosulfophthalein, bile acids, steroid hormones, etc. Since PGT, a transporter capable of transporting prostaglandins, also shows significant homology to OATP-A, genes encoding these transporters are thought to form a gene family (the OATP family).
Only a few reports are available on identifications of transporters at the molecular level, including above reports, and it is believed that many unidentified transporters exist that can be clinically useful.